Light barriers are frequently used for the contactless recognition of objects. The optical spectrum is, however, not suitable for all applications, for instance in environments with severe contaminants. There are therefore areas of use in which a microwave barrier represents the more favorable choice. Microwaves not only penetrate through opaque small particles, but also through at least thin films of non-metallic objects and thus even allow a monitoring in closed containers. A microwave barrier in principle works like a light barrier: A microwave signal is transmitted at one side of the zone to be monitored and is received again at the opposite side. If an object enters into this microwave path, the received microwave signal changes and the presence of the object can be recognized by this.
A specific use for microwave barriers is the point level detection of media such as liquids or bulk goods in containers. The object to be recognized here is the medium which reaches the microwave path from a certain filling level or point level onward and thereby damps the microwave signal. The critical filling levels of overflow or no-load running are thus in particular monitored so that a regulation of the filling level can intervene in good time. A number of different alternatives are known for a point level determination, for instance tuning fork sensors, radar sensors, systems based on ultrasound or capacitive systems. Microwave barriers show advantages over these with respect to the robustness toward interference influences such as temperature, pressure, dust or noise.
Conventional microwave barriers, such as are known from DE 33 02 731 A1, are divided into a transmission unit and a reception unit. This first means that the microwave barrier comprises two different units which cannot be swapped over. In addition, in many respects, the association between the transmission unit and the reception unit must be ensured, above all when a plurality of microwave barriers are operated next to one another. For instance, an antenna design having a small transmission lobe is selected to achieve an alignment effect on the associated, oppositely disposed unit of the microwave barrier. Such antennas can take on large dimensions, which are disturbing in the assembly, and a very precise assembly and alignment of the units is simultaneously required.
In addition, parameters such as the carrier frequency, the modulation frequency or the polarization direction in the transmission unit and in the reception unit also have to be set so that the associated units recognize one another and are influenced as little as possible by external units of other microwave barriers. Such settings can only be made with an exact understanding of the operation of the microwaves and the required time effort for putting into operation is even increased for corresponding technical personnel. The named restrictions for ensuring an unambiguous association also have an effect on the arrangement options of the units which should, for example, have sufficient distance from one another to avoid incorrect associations. Since there is no higher ranking technical information link between the conventional transmission unit and reception unit, interference influences such as a reducing transmitter power or multipath propagation are difficult to control.